Simulation of the Control of a Salt Gradient Solar Pond in the South of Tunisia

We are interested in modelling and control of a salt gradient solar pond (SGSP) in the south of Tunisia. We developed a model of a Closed Cycle Salt Gradient Solar Pond (CCSGSP) that ensure successful year round operation of the pond. This model was used to study the response of solar pond (SP) to various control technique. It takes into account heat and salt diffusion in the pond and simulates the transient behaviour of a SGSP. Furthermore, we investigated the dynamic process, which involve internal gradient stability, boundary behaviour between gradient zone and convective zones. We thus incorporated the double diffusive processes into the SP model by using the one dimensional stability criterion produced by linear theory. The governing differential equations are solved with a fully implicit technique described by Patankar. The results show that successful operation of a SP requires three things: the maintenance of the storage zone temperature through heat extraction and brine injection, the use of surface washing to control the deepening of the upper mixed layer and a well designed initial salt stratification to prevent the formation of instability within the gradient. Using linear salinity profile as an initial condition, three-year simulations were run using average meteorological data with the result that adequate stability ( Rρ > 2 throughout the gradient and Rρ ≅ 10 at the interfaces) was maintained and during the entire year. Numerical results show also that 15 % 30 % efficiency could have been expected if heat extraction is performed routinely especially when one considers that the storage temperature is within 40 80 °C.